add a value to a dropdown custom field from a user input

[Jacob francois]

add a value to a dropdown custom field from a user input in another custom field.

[Nic Brough -Adaptavist-]

This is usually something your admins should be doing, not letting anyone do. 

But, have you tried using a label field instead?

[Jacob francois]

our dropdown field already has 700 values. is there a way to add them to labels ?

[Nic Brough -Adaptavist-]

If you have one of the scripting options you mention in the question tags, then yes, you should be able to write a short script to read all the issues that have a value set and add the value as a label to the current issue.

https://library.adaptavist.com/entity/bulk-update-the-value-of-a-custom-field-on-jira-issues might help you get started on a script.